This thesis demonstrates the feasibility of using gallium nitride (GaN) technology in reconfigurable RF systems. GaN-based varactor diodes and switch circuits are pursued as promising candidates for high-power/high-frequency applications. The first part is devoted to active GaN device development. Active components were realized using the Canadian National Research Council (NRC) GaN HEMTs process. Based on three process, such as, GaN150v0 (gate length of 0.15um), GaN500v1 and GaN500v2 (both with gate length of 0.5um), many varactor diodes with size different have been manufactured and characterized via DC and RF small-signal and large-signal measurements. Then, the varactor diodes were modeled by analytic equations containing empirical coefficients. These expressions have been introduced for the first time for the voltage dependency of equivalent capacitance (CEq) and series resistance (REq) and can be used as a general model to represent the nonlinear behavior of GaN based varactors. For small-signal operation, all of the developed equations describing REq and CEq are only bias voltage and device geometry dependent, while for large-signal operation, the influence of RF-power must be taken into account. In addition, different size single stand-alone switches were fabricated using GaN500v2. By analyzing the small-signal measurements, it was observed that the isolation is high at low frequencies but quickly drops with increasing frequency. Also, it was observed that the larger the device the lower the insertion loss and the poorer the isolation will be. Moreover, based on these small-signal measurements, a model was introduced. The second part addresses the integration and design aspects of the reconfigurable proposed circuits, such as tunable phase shifter, reconfigurable 3-dB 90° hybrid coupler, tunable frequency oscillator, beam switching antenna array and matching reconfigurable patch antenna operating below 10 GHz. Here the developed GaN varactors and switches are used for achieving the tenability purpose.